The present invention relates in general to a method of detecting ink concentration and ink jet recording apparatus, and more particularly to a method and recording apparatus of detecting ink concentration of ink to be supplied to an ink jet head.
An electrostatic ink jet recording system is known as a kind of ink jet recording system for ejecting ink towards a recording medium to record a desired image on the recording medium.
In the electrostatic ink jet recording system, used as ink is an ink composition which is obtained by dispersing charged color particles into a dispersion medium (hereinafter referred to as “ink” for short), and the ink is supplied from an ink tank to an ink jet head to be ejected in the form of minute ink droplets from a large number of ejection portions formed in the ink jet head. When the ink is ejected from the large number of ejection portions, predetermined voltages are respectively applied to electrodes provided in each of the ejection portions of the ink jet head. As a result, electrostatic forces are generated in the ejection portions. Then, the color particles contained in the ink are highly concentrated in the ejection portion by the electrostatic force, and an ink droplet containing the highly concentrated color particles is flown towards a recording medium to strike on the recording medium. In such a manner, a desired image is recorded on the recording medium.
In the electrostatic ink jet recording system, as described above, the color particles contained in the ink are highly concentrated in the ejection portion by the electrostatic force. Hence, repeated ejection of the ink reduces an amount of color particles contained in the ink. When an amount of charged color particles within the ink decreases, there is a fear that the color particles are hardly highly concentrated by the electrostatic force, which reduces a frequency at which the ink droplet is ejected from the ejection opening, making it impossible to form a desired image. In order to avoid such a situation, normally, an ink tank is suitably replenished with high concentration ink so that the ink concentration becomes constant.
Examples of a recording apparatus in which ink concentration is detected and an ink tank is replenished with high concentration ink based on the detected ink concentration include an electrostatic ink jet recording apparatus disclosed in JP 2,834,100 B, in which an amount of toner particles contained in ink passing through a pipe is detected by magnetic or optical means, it is judge whether or not the amount of toner particles is enough to realize the sufficient printing concentration, and an ink tank is replenished with the toner particles when the amount of toner particles is less, i.e., is not enough to realize the sufficient printing concentration.
However, in the recording apparatus including a device for detecting ink concentration, a component such as a detection element or a detection cell used to detect the ink concentration contacts ink. Thus, the ink is adhered to such components, and for example, after a long period of time has elapsed, the color particles contained in the ink are firmly adhered to such components. As a result, a problem arises in that even though the ink concentration is actually low, the detected ink concentration is higher than the actual ink concentration by an amount of color particles firmly adhered to an inner wall of the detection cell. When a difference occurs between the actual ink concentration and the detected value obtained by the detection element, it becomes impossible to form an image on the recording medium at desired concentration, and thus the image quality is reduced.